This invention relates to rotational energy absorbing couplings, and more particularly to shaft-to-crank arm coupling for a tiller arm. The device will absorb part of the energy of impact of fully submerged steerable strut-foil system of hydrofoil ships which may encounter submerged logs, large fish, and debris. These impacts have often resulted in steering actuator failures which then produced large rolling and yawing motions as a result of uncontrolled forward strut rotation. After the steering actuator fails only limit stops would prevent strut-foil rotation.
Studies conducted on hydrofoil ships determined that the debris impact load was impulsive in nature, and there was no provision of energy absorbing devices. Therefore, absorption of the foil-strut rotational energy, built up during the impact, would have to be the primary function of a device to prevent failure of the steering actuator. It was found that if the actuator did fail, high lateral bending loads could be experienced by the strut due to hydrodynamic loads as a result of large rotations. These high lateral bending loads in the strut can present significant hazards to the ship and personnel, for example the strut may break off. Prior art rotational couplings generally use shear pin arrangements which are bulky and which merely shear to avoid catastrophic damage to other components, but do not absorb energy to any particular degree before their total failure in shear. They also require immediate replacement and repair before continued operation.